It has evacuated tubes but instead of being the heat pipe version, they actually have the water running through them. I'm planning on using it as a stand alone panel with a thermosyphon tank above it connected to the manifold of the panel. I'm currently building a tiny house on a trailer and there is not enough room indoors for a tank and pump hot water system so this will be outside on a stand, operating more like a batch heater than a continuous cycle system. When the water in the tank is hot enough it can be drawn into the house by a short pipe.

I'm considering using an old hot water tank mounted above the panel and connected to give a thermosyphon effect. A standard, insulated, copper hot water tank is around 120 liters which I think should be about the right size, but I'm not sure of the best way to connect the tank, as I understand most of them have heat exchange coils inside.

I'd be grateful of any suggestions on the best way to approach this or any alternative ideas that you feel might work better. I'm doing this on a very tight budget as the whole tiny house is being built from recycled and second-hand materials.

Hot water tanks are usually lined with cast glass since its nearly inert. An electric hot water tank is nearly perfect to convert to a batch heater.

Interior heat exchangers are possible to retrofit in but a more common practice is to spiral wind the exterior of the cast glass or glass lined tank and then put the original insulation and covers back on.

Since this is going into a trailer how about a custom sized tank from plywood and roofing membrane that would occupy some sturdy spot you've reinforced with steel connected into the trailer subframe? The space left under a twin sized bed will hold a lot of water!

Without seeing the exact details of the solar panel I don't know if it will thermosiphon or not.

In a thermosiphon system, all the water flow must be smooth and always gravity feed.If there is any area within the panel that doesn't flow slightly downward, the thermosiphon system can lock up. Of course, if it doesn't a pump can be added.

I'm based in London, England. The panel I have should be able to thermosyphon, as the company that makes it makes a version popular in China that does exactly that. The difference is that instead of the tubes going into a manifold, they go directly into a tank built onto the top of the panel, as you can see in the version here:

In this style of panel the water naturally thermosyphons down the side of the tubes and back up into the main tank. My plan is to have a tank above the panel where the in and out from the manifold connect to the tank. I figured I might have to experiment with putting the panel on an angle to give gravity a bit of a hand.

Alternatively I could find an old tank and see if I could use the fittings from the manifold and fit them directly into the wall of the tank, but that would probably lead to leakage unless it's done really well.

I didn't want to fit a tank inside the house as I'm having to build down to a low weight and I'm very tight on space and budget. I did consider a tank in a corner but I'd have to drain the system each time I move and I'm going to be using rainwater for my water supply, not connected to the mains. Also, that would mean putting the panel on the roof of the cabin, and I'm not sure if the panel would survive being driven down the highway unless I take the tubes out each time I move.

Now, I'm not intending to move often, it's not an RV, more of a moveable home. If I went for a regular type of system, I guess I'd need a 130 liter or so insulated tank (is there a way to figure out the minimum?) and a 12v circulation pump. Does the pump have to be on constantly or just occasionally. My power supply for the cabin will be a 12v 220ah battery pack charged from two 85w pv panels so I'd need to keep things as simple and low power as possible.

Ron, thank you for the link to that book, I shall be reading my way through it over the next few days.

OK the basics of a thermosiphon system are rather simple, the collector is the lowest part of the system, and all cold water runs downhill and all hot water runs uphill. there must be some gradient in all parts of the system to ensure that the water constantly flows.

The pipes must always have some gradient to them in all the system and there must not be any dips in either side. If for instance the hot side line exits the collector then goes lower than the outlet, the thermosiphoning can fail,because you've created a high point that the water doesn't want to flow past.

As far as heat exchanging in them you can run a series of coils in the tank which is probably the most efficient, or wrap the tank with the coils, once again there can be no dips in these coils! You may also consider glazing your tank cover and have some heat gain as in a batch system, but if the solar gain is greater in the tank than the collector, thermosiphoning will fail.

Even though a thermosiphon system seems simpler it is not always as efficient. A pump properly regulated can ensure better heat transfer.

If you are in the UK then pretty much any gas fired back boiler hot water tank will do the job.

These older style tanks have a 22 or 28mm coil inside for thermosyphoning from the boiler downstairs to the hot water tank upstairs in the airing cupboard!! Newer tanks use a smaller diameter indirect coil as they are pumped rather than thermosyphon.

In the UK, the tank will be all copper or an alloy rather than the 'enamelled' (glass lined) tanks common here in NZ and in Aus and will have a 50mm foam insulation over it.

It will also have a large hole for an electric emersion heater in the 3-5kw range for use in the summer when the main boiler isn't being used.

Talk to your local plumber about an old one but don't get one in London - the water there is very 'hard' and you will be buying 100lb of chalk in the bottom of the tank and a 1 inch thick coating of limescale on the internal coil - try the West country or Wales where the water is 'soft'.

A lot of people are replacing there old boilers with the combi instant water types which mean the hot water tank is no longer required and are removed by the plumbers.

The plumbers usually take them down to the local metal dealers to cash in on the copper.

I was lucky and obtained my tank and most of the fittings as well from a plumber.I gave him a couple of £ and he was happy.

My system is thermosyphoning to a solar tank in my loft which then drains into my main hot water tank in the airing cupboard.

I am assuming that when you say you are building a house on a trailer it is not going to be towed as any sizable tank at height will have an effect on the stability if towed. As long as the tank is above the collector it will work. The head of water in the tank will determine the ouput pressure to the taps if direct feed.